High entropy alloy (HEA) nanoparticles hold promise as active and durable (electro)catalysts. Understanding their formation mechanism will enable rational control over composition and atomic arrangement of multimetallic catalytic surface sites...

Understanding the nanoscale water condensation dynamics in strong electric fields is important for improving the atmospheric modeling of cloud dynamics and emerging technologies utilizing electric fields for direct air moisture capture. Here, we use vapor-phase transmission electron microscopy (VPTEM) to directly image nanoscale condensation dynamics of sessile water droplets in electric fields. VPTEM imaging of saturated water vapor stimulated condensation of sessile water nanodroplets that grew to a size of ∼500 nm before evaporating over a time scale of a minute. Simulations showed that electron beam charging of the silicon nitride microfluidic channel windows generated electric fields of ∼10 8 V/m, which depressed the water vapor pressure and effected rapid nucleation of nanosized liquid water droplets. A mass balance model showed that droplet growth was consistent with electric field-induced condensation, while droplet evaporation was consistent with radiolysis-induced evaporation via conversion of water to hydrogen gas. The model quantified several electron beam−sample interactions and vapor transport properties, showed that electron beam heating was insignificant, and demonstrated that literature values significantly underestimated radiolytic hydrogen production and overestimated water vapor diffusivity. This work demonstrates a method for investigating water condensation in strong electric fields and under supersaturated conditions, which is relevant to vapor−liquid equilibrium in the troposphere. While this work identifies several electron beam−sample interactions that impact condensation dynamics, quantification of these phenomena here is expected to enable delineating these artifacts from the physics of interest and accounting for them when imaging more complex vapor−liquid equilibrium phenomena with VPTEM.

High entropy alloy (HEA) nanoparticles hold promise as active and durable (electro)catalysts. Understanding their formation mechanism will enable rational control over the atomic arrangement of multimetallic catalytic surface sites. While prior reports have attributed HEA nanoparticle formation to nucleation and growth, there is a dearth of detailed mechanistic investigations. Here we utilize transmission electron microscopy (TEM), systematic synthesis, and mass spectrometry (MS) to demonstrate that HEA nanoparticles form by aggregation of non-crystalline multimetallic cluster intermediates. AuAgCuPtPd HEA nanoparticles were synthesized by aqueous co-reduction of metal salts with sodium borohydride in the presence of thiolated polymer ligands. Varying the metal:ligand ratio during synthesis showed that alloyed HEA nanoparticles formed only above a threshold ligand concentration. Alloyed sub-nanometer clusters were observed with the final HEA nanoparticles while few clusters were observed when phase-separated nanoparticles formed. Increasing supersaturation ratio increased particle size, which together with the observations of stable single atoms smaller than the critical nuclei size was inconsistent with a burst nucleation mechanism. Direct real-time observations with liquid phase TEM imaging showed that HEA nanoparticles formed by aggregation of sub-nanometer clusters. Taken together, these results are consistent with a reaction mechanism involving rapid reduction of metal ions into sub-nanometer alloyed clusters, followed by cluster aggregation driven by borohydride ion induced thiol ligand desorption. This work suggests intermediate cluster species as potential synthetic handles for rational control over HEA nanoparticle atomic structure.

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Assembling 2D‐material (2DM) nanosheets into micro‐ and macro‐architectures with augmented functionalities requires effective strategies to overcome nanosheet restacking. Conventional assembly approaches involve external binders and/or functionalization, which inevitably sacrifice 2DM's nanoscale properties. Noble metal ions (NMI) are promising ionic crosslinkers, which can simultaneously assemble 2DM nanosheets and induce synergistic properties. Herein, a collection of NMI–2DM complexes are screened and categorized into two sub‐groups. Based on the zeta potentials, two assembly approaches are developed to obtain 1) NMI‐crosslinked 2DM hydrogels/aerogels for heterostructured catalysts and 2) NMI–2DM inks for templated synthesis. First, tetraammineplatinum(II) nitrate (TPtN) serves as an efficient ionic crosslinker to agglomerate various 2DM dispersions. By utilizing micro‐textured assembly platforms, various TPtN–2DM hydrogels are fabricated in a scalable fashion. Afterward, these hydrogels are lyophilized and thermally reduced to synthesize Pt‐decorated 2DM aerogels (Pt@2DM). The Pt@2DM heterostructures demonstrate high, substrate‐dependent catalytic activities and promote different reaction pathways in the hydrogenation of 3‐nitrostyrene. Second, PtCl4 can be incorporated into 2DM dispersions at high NMI molarities to prepare a series of PtCl4–2DM inks with high colloidal stability. By adopting the PtCl4–graphene oxide ink, various Pt micro‐structures with replicated topographies are synthesized with accurate control of grain sizes and porosities.